This invention relates to a throwaway insert capable of machining surfaces that cross each other at an angle of substantially 90.degree. (such machining is hereinafter referred to as perpendicular machining) on a workpiece, and a cutting tool equipped with such inserts, particularly a rotary cutter such as a face milling cutter, end milling cutter or side cutter. But the throwaway insert of the invention may also be mounted on tools for turning.
For perpendicular machining, triangular throwaway inserts and parallelogrammic ones as shown in FIG. 8 at 21 are used. These inserts have only three and two usable corners, respectively. Substantially square inserts having four usable corners are also known. E.g. in Unexamined Japanese Patent Publication 7-246505, there is disclosed an insert which has a main and an auxiliary cutting edge at each corner and is capable of finish-machining a bottom surface with the auxiliary cutting edge. As viewed from one side, each auxiliary edge is inclined inwardly at a very small angle relative to the main cutting edge. The flank of each main cutting edge comprises a first and a second face, the first face being inclined at a smaller angle than the flank of the auxiliary cutting edge. This substantially square throwaway insert can be used for perpendicular machining. A throwaway insert shown in FIG. 9 is also known.
The conventional substantially square throwaway insert having auxiliary edges can machine a side and a bottom that cross each other at substantially 90.degree.. But such a throwaway insert of this type can be used only when the cutting tool is rotated in one direction. On the tool which is rotated in the opposite direction, this insert has to be replaced with another insert having its main and auxiliary edges arranged in opposite ways. If as viewed from one side of the insert, the main edge extends obliquely downward toward the center, when the depth of cut is larger than half the length of each main cutting edge, a force that pushes the workpiece upward will be produced. This force causes chattering or lowers machining accuracy.